1. Field of the Invention
The present invention relates to a cored motor, particularly, relates to a cored motor having a dynamic pressure bearing, which is suitable for driving a disc installed in a hard disc drive (HDD).
2. Description of the Related Art
A typical example of a conventional cored motor is disclosed in the Japanese publication of unexamined patent applications No. 10-4642/1998. The cored motor disclosed in the publication is composed of a shaft, a stator case, a rotor core that is installed with laminated cores, and a magnet that is mounted on the inner surface of the stator case so as to confront with the outer circumferential surface of the rotor core.
Further, there existed another example of a cored motor for HDD according to the prior art shown in FIGS. 4 and 5.
With referring to FIGS. 4 and 5, the cored motor for HDD according to the prior art is explained.
FIG. 4 is a cross-sectional view of a cored motor according to the prior art.
FIG. 5 is a fragmentary cross-sectional view, with enlarging a part of the cored motor indicated by a circle “B” in FIG. 4.
In FIGS. 4 and 5, a cored motor is composed of a stator 103 and a rotor 121 that is provided with a rotor hub 102, which is installed with a disc 26.
The stator 103 is further composed of a motor base 113, a stator core 114 that is fixed to the motor base 113, and a sleeve 109.
The sleeve 109 sustains a shaft 101 through a dynamic pressure bearing so as to rotate freely as well as being fixed to a center hole 113a of the motor base 113, wherein description of the dynamic pressure bearing is omitted.
On the other hand, the rotor 121 is further composed of the rotor hub 102, an annular rotor yoke 105, and a ring magnet 106. The rotor 121, annular rotor yoke 105 and ring magnet 106 are assembled such that the annular rotor yoke 105 is fixed to an outer circumferential area of the rotor hub 102 on the bottom and the ring magnet 106 that is electro-painted is affixed on the inner circular side surface of the annular rotor yoke 105. In this assembling process, as shown in FIG. 5, positioning of the ring magnet 106 in an axial direction along the shaft 101 is limited by a stopper section 117, wherein the stopper section 117 is provided on an inner circumferential area of the annular rotor yoke 105.
Further, the shaft 101 is press-fitted into the rotor hub 102 and the rotor 121 enables to rotate freely with respect to the stator 103.
A section of the annular rotor yoke 105 is formed in an inverted “L” shape having a crooked section 112 in the rotor hub 102 side, wherein the crooked section 112 extends toward a center of the annular rotor yoke 105. The crooked section 112 is formed by the drawing process.
Further, the crooked section 112 effectively protects a recording and reproducing head 150 of an HDD from being affected by magnetic flux possibly leaking from the ring magnet 106.
Furthermore, a clearance section 108 is provided between the ring magnet 106 and the annular rotor yoke 105 so as to reduce magnetic short-circuiting by reducing a contact area between them. The clearance section 108 is also provided for relieving a caulking jig when a caulking process is applied to the rotor hub 102.
With referring to FIG. 5, a positional relationship in the axial direction among the rotor yoke 105, the ring magnet 106, and the stator core 114 is detailed next.
In FIG. 5, the ring magnet 106 and the stator core 114 is allocated such that a center line 118 of the ring magnet 106 having a length L102 in the longitudinal direction along the shaft 101 is shifted upward by a distance DL with respect to another center line 119 of the stator core 114 having a thickness L103 in the longitudinal direction along the shaft 101. Shifting the center line 118 of the ring magnet 106 upward is caused by canceling shift of a magnetic center of the rotor 121 side. If the center lines 118 and 119 coincide with each other, by the crooked section 112 of the rotor yoke 105, a magnetic center of the rotor 121 side is shifted downward with respect to a magnetic center in the longitudinal direction along the shaft 101 of the stator core 114.
In the meanwhile, as mentioned above, the conventional cored motor was provided with the crooked section 112 on the rotor yoke 105 in order to prevent the recording and reproducing head 150 from being affected by magnetic flux possibly leaking from the ring magnet 106.
Further, magnetic flux density of the ring magnet 106 was diminished by the stopper section 117, which short-circuited a part of the magnetic circuit in the rotor hub 102 side. Therefore, as mentioned above, the clearance section 108 was provided so as to relieve a certain level of diminishing magnetic flux density.
However, it was necessary for a forming process of the rotor yoke 105 to provide an extra process for forming the crooked section 112 and the clearance section 108, and resulting in increasing cost of component parts.
Further, since the crooked section 112 was formed by the drawing process, there existed another problem in the manufacturing process such that forming the crooked section 112, the stopper section 117, and the clearance section 108 in higher dimensional accuracy was extremely difficult.
In the mass production of the conventional cored motor, dimensional accuracy of positioning the ring magnet 106 and the rotor yoke 105 fluctuated extremely. In this connection, magnetic flux density was deviated to the upper side of the ring magnet 106, toward the rotor yoke 105, or to the lower side of the ring magnet 106, toward the motor base 113. Consequently, it was difficult to coincide the magnetic center of the ring magnet 106 with the magnetic center of the stator core 114 accurately and without any deviation.
In this case, there existed a further problem such that the rotor hub 102 has rotated with being sucked in either axial direction of upward or downward, and resulting in making vibration and noise larger due to unbalanced rotation of the rotor hub 102.
Further, in the case that a posture of an apparatus installing the conventional cored motor is indefinite, balanced revolution of a rotor hub of a cored motor is intentionally unbalanced by making the rotor hub to be sucked in the axial direction so as to reduce vibration and so as not to degrade revolving efficiency in spite of the posture of the apparatus. Such a cored motor was disclosed in the Japanese publication of unexamined patent applications No. 10-4642/1998.
However, particularly, in the case that such a cored motor is installed in a HDD, actually, posture of the HDD is almost fixed. Consequently, it is most important for the conventional cored motor to balance the revolution of the rotor hub of the cored motor by coinciding the magnetic center of the ring magnet with the magnetic center of the stator core.
Further, in the case that a bearing of the cored motor is a dynamic pressure bearing, there exists a furthermore problem in connection with reliability such that unbalanced force in the axial direction, which occurs when the balanced revolution of the rotor hub collapses, results in shortening life of the dynamic pressure bearing extremely.